Field of the Invention
The present invention relates to a boundary microphone having a sound collecting characteristic independent from installation locations or sound collecting angles, and a boundary plate used for this boundary microphone.
Description of the Related Art
Boundary microphones (microphones for collecting sound on a plane) are often used in TV studios or at meetings. In TV studios, on stages, or in concert halls, boundary microphones are used while placed on a floor. At meetings, those microphones are used while arranged on a table.
These boundary microphones include, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 8-65786 and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-527995, a boundary plate and a microphone placed thereon. Usually, the boundary plate is a plate with a flat and reflecting surface made of a metal or plastic.
Incidentally, when a microphone alone is used, direct sound as well as reflected sound (indirect sound) reach the microphone. Since the reflected sound arrives with a delay compared with the direct sound, the reflected sound interferes with the direct sound, thereby deteriorating the intelligibility of sound signals.
Meanwhile, although a boundary microphone collects direct sound and indirect sound reflected by the boundary plate, the microphone is arranged in proximity of the boundary plate, thus allowing for collecting the direct sound and the indirect sound with little difference in time.
Therefore, sound signals with high intelligibility, free from interference from the delayed reflected sound, can be obtained from the microphone. It is also known that an output sound level of the microphone can be increased since the microphone receives the direct sound as well as the indirect sound reflected by the boundary plate.
FIGS. 1 and 2 are a top view and a front view, respectively, showing a basic configuration of the aforementioned boundary microphone.
A boundary microphone 1 shown in FIGS. 1 and 2 includes a boundary plate 2 formed in a rectangular shape and, for example, a condenser microphone 3 placed thereon. The boundary plate 2 shown in this example has long sides (sides in the longitudinal direction of the condenser microphone 3) of 300 mm and short sides (sides in the direction perpendicular to the longitudinal direction) of 200 mm.
A front end of the condenser microphone 3, namely, a front acoustic terminal of the condenser microphone 3, is positioned at, for example, 80 mm from the end in the longitudinal direction of the boundary plate 2 (right-side end in FIGS. 1 and 2) and in the center part in the short side direction such that a sound collecting axis of the microphone 3 is parallel to the top surface of the boundary plate 2.
Note that, although not shown, this boundary microphone 1 including the boundary plate 2 and the condenser microphone 3 placed thereon is housed in a flat casing having a punched plate (perforated plate) covering the whole structure.
In FIG. 2, the sound collecting axis of 0° of the condenser microphone 3 as well as directions (angles) of sound sources seen from this sound collecting axis are shown. Hereinafter, based on a relationship between the condenser microphone 3 and directions (angles) of a sound source as shown in FIG. 2, respective characteristics shown in FIG. 3 and the subsequent drawings are described.
FIGS. 3A to 3C show measured results of characteristics of the unidirectional condenser microphone 3 alone when the boundary plate 2 is not used therewith. These characteristics of the microphone alone are utilized for comparison with a case where a boundary plate of the related art or a boundary plate according to an embodiment of this invention is used. Both boundary plates will be described later.
That is, FIG. 3A shows frequency characteristics expressed by the horizontal axis depicting frequency and the vertical axis depicting output level (dBV), as is well known. In FIG. 3A, symbols A, B, C, and D indicate measurement results for cases where a sound source is placed at angular positions of 0°, 90°, 180°, and 270°, respectively, relative to the sound collecting axis of the microphone 3.
Likewise, FIG. 3B shows frequency characteristics where symbols A, B, and C indicate measurement results for 0°, 30°, and 40°, respectively.
Furthermore, FIG. 3C shows a polar pattern. As shown in this polar pattern, characteristics of the exemplified condenser microphone 3 alone show general unidirectional characteristics.
Next, in FIGS. 4A to 4C, respective characteristics of the boundary microphone 1 are shown for the boundary plate 2 of the related art made of, for example, plastic and not transmitting sound waves. Dimensions of the boundary plate 2 and arrangement of the condenser microphone 3 are as exemplified in FIGS. 1 and 2.
Meanwhile, in FIG. 4A, frequency characteristics are shown where symbols A, B, C, and D indicate measurement results for angular positions of 0°, 90°, 180°, and 270°, respectively, relative to the sound collecting axis of the microphone 3. Likewise in FIG. 4B, symbols A, B, and C indicate measurement results for 0°, 30°, and 40°, respectively, and FIG. 4C shows a polar pattern.
Note that, upon measurements for FIGS. 4A to 4C, the condenser microphone 3, with which the measurement results shown in FIGS. 3A to 3C described above have been obtained, is used as the boundary microphone.
Here, when comparing a frequency characteristic at 90° indicated by symbol B in FIG. 4A and a frequency characteristic at 90° indicated by symbol B in FIG. 3A where the condenser microphone 3 alone is used, the frequency characteristic indicated by symbol B in FIG. 4A shows irregular peaks and dips for a wide range of frequency bands.
This is because the plastic boundary plate itself vibrates upon receiving sound waves from the direction perpendicular to (90°) a surface of the boundary plate 2. This free vibration of the boundary plate 2 causes phase interference of sound signals.
Also, frequency characteristics at 30° and 40° indicated by symbols B and C, respectively, in FIG. 4B show increased levels over the frequency bands of 500 Hz to 6 kHz when compared with a characteristic at 0° indicated by symbol A in FIG. 4B. The amount of increase reaches 6 dB or more.
This is because, by attaching the aforementioned plastic boundary plate 2 not transmitting sound waves, reflected waves in the frequency bands of 500 Hz to 6 kHz also reach the microphone 3, thereby increasing the levels.
In other words, sound in the aforementioned frequency bands is reproduced while stressed at an angle of 30° or 40°, providing sound signals largely different from the original sound.
Therefore, when the boundary plate of the related art made of plastic or metal and not transmitting sound waves is used, different directional frequency responses are experienced depending on installation locations of the boundary microphone, thus providing different tones depending on angles of the sound source.
For these reasons, when a microphone provided with the aforementioned boundary plate in the related art is used, for example, for collecting sounds of musical instruments, tones may vary depending on locations of the musical instruments, which is not preferable for collecting sound of good sound quality such as musical sound.